1. Field of the Invention
The invention relates to denture adhesives and to processes for making and using denture adhesives.
2. Description of Related Art
Dentures are substitutes for missing teeth and serve as a replacement for all or some of the teeth found in the oral cavity. Despite diligent efforts by dental professionals and designers of dental prostheses, dentures do not always fit perfectly. Over time, even well-fitting dentures can become ill-fitting due to natural shrinkage and changes in the gum or mucous tissues. Therefore, adherent creams, liquids or powders are often used to secure or temporarily fix dentures within the mouth.
There are a number of desirable attributes of such a denture adhesive composition. The denture adhesive should develop a high degree of tack upon contact with saliva so that the dentures can be held in place as soon as they are seated in the mouth. It is also highly desirable that the saliva hydrated adhesive be spread over the denture-mucosa interface in order to seal the denture in place effectively. The mucilage should possess sufficient cohesive strength to withstand the stresses of mastication, which act to rupture the seal and thus dislodge the denture. The denture adhesive must also exhibit sufficient resistance to degradation under the extreme environmental changes that can occur in the oral cavity during such common actions as drinking hot or cold beverages. Of course, the adhesive must also be releasable so that the denture wearer may remove the dentures for cleaning and maintenance. Denture adhesives are generally sold as a cream, liner or strip, liquid or powder, and many examples are well known in the art.
A major factor in selecting one denture adhesive over another in the marketplace, however, remains the holding performance of a denture adhesive. Thus, workers in the field have been constantly seeking better holding performance.
Early denture adhesives contained finely ground particles of natural gums that expanded when wet to become a viscous gel, which acted as a cushion and an adherent between a denture plate and gum tissue. These denture adhesives, however, have been largely supplanted by polymeric denture adhesives in recent years.
A popular current choice for a polymeric denture adhesive comprises a methyl vinyl ether/maleic anhydride copolymer known as "Gantrez.RTM.." This polymer is combined with a number of other ingredients to form the final denture adhesive.
Polymers, of course, have been known for many years and have been used for many different purposes. Depending on the monomer or monomers selected, a polymer may have a vast array of properties and be useful for a number of different purposes. Once the basic constituents of the polymer or copolymer have been selected, any number of modifications may be made to affect various properties of the final polymer or copolymer. The chain length of the polymer may be regulated to be short or long, and, in the case of copolymers of two or more chemically distinct monomers, the order of the monomers may be regulated to form a block copolymer or an alternating copolymer or anything in between.
Once the structure of the polymer or copolymer has been defined, the properties of the polymer or copolymer may still be modified significantly by various chemical or physical methods. One method of modifying the properties of a polymer or copolymer is to modify various ligands that may be present on the polymer chain. For example, with a MVE/MA copolymer, the anhydride group may be hydrolyzed to the corresponding dicarboxylic acid. The carboxylic acid, in turn, may be fully or partially neutralized to form a salt or the carboxylic acid may be fully or partially esterified with various groups. The modification of the carboxylic acid leads to a change in the properties of the copolymer. The copolymer may be plasticized or otherwise modified by the addition of a cation or covalently bound ligand.
These ionic or covalent bonds may lead to crosslinking of the copolymer chains. If a salt is formed with a divalent cation, the cation may form an ionic "cross-link" between two copolymer chains. Such a "cross-link" may be very easily dissociated in the presence of water. If a compound contains two or more groups capable of forming an ester with a carboxylic acid ligand, then covalent cross-links may be formed between adjacent copolymer chains. These covalent cross-links may have more or less resistance to hydrolysis depending on the ligand chosen.
Polymers and copolymers of anhydrides, including maleic anhydride, have been known for a long time and have been used for many purposes. For example, U.S. Pat. No. 2,313,565 to McDowell et al., issued Mar. 9, 1943, is directed to a copolymer of an acid anhydride with a vinyl ether monomer. The anhydride may be treated with ammonia or a primary or secondary amine to form an imide. The copolymer is useful as a sheeting or film base.
Anhydride copolymers have also been used as carriers for sustained release pharmaceuticals, at least where the pharmaceutical could be used as an amine salt. In U.S. Pat. No. 3,121,043 to Tobin et al., issued Feb. 11, 1964 the anhydride is lightly cross-linked with the pharmaceutical salt, and the active pharmaceutical is released over time as the amine link is hydrolyzed. A copolymer of maleic anhydride and methotrexate for treating tumors is discussed in U.S. Pat. No. 4,182,800 to Ringsdorf et al., issued Jan. 8, 1980.
Anhydride copolymers may also be used in hair spray or cosmetics (U.S. Pat. No. 3,974,128 to Block et al., issued Aug. 10, 197G and U.S. Pat. No. 5,753,215 to Mougin et al., issued May 19, 1998); as thickening agents (U.S. Pat. No. 3,684,776 to Field et al., issued Aug. 15, 1972 and U.S. Pat. No. 3,878,151 to Dachs et al., issued Apr. 15, 1975); and as anti-static agents for flammable liquids (U.S. Pat. No. 3,729,452 to Andress et al., issued Apr. 24, 1973). Mougin discusses MVE/MA copolymers monoesterified with butanol and neutralized by diamines or polyvalent metal salts. The diamines include lysine, arginine and cystine, and the polyvalent metal salts include bromides, chlorides, nitrates, acetates, carbonates and sulphates of calcium, zinc, magnesium, barium, aluminum and zirconium.
Anhydride copolymers have been used to support biological molecules in U.S. Pat. No. 5,760,166 to Charles et al., issued Jun. 2, 1998. The copolymer has a nitrogen or sulfur ligands such as ethanolamine or mercaptoethanol on some of the carboxyl groups. Anhydride copolymers have also been used to precipitate proteins from aqueous media in U.S. Pat. No. 5,534,597 to Krupey, issued Jul. 9, 1996 and U.S. Pat. No. 5,294,681 to Krupey, issued Mar. 15, 1994. The copolymer is a water insoluble cross-linked polyhydroxy polycarboxylic acid having at least two strands linked by at least one diamine cross-linking agent.
A gel-forming system useful in wound dressings contains a mixture of polymers as discussed in U.S. Pat. No. 5,578,661 to Fox et al., issued Nov. 26, 1996. The system has a water-soluble polymer, such as polyethylene oxide, an acid containing polymer, such as a MVE/MA copolymer, and an amino-containing polymer, such as polysaccharides or poly-L-lysine. Another gel is discussed in U.S. Pat. No. 5,521,256 to Kwak et al., issued May 28, 1996. Sodium hydroxide is added to a crosslinked MVE/MA copolymer, thereby forming a gel solution.
Denture adhesives are not the only adhesive materials that contain anhydride copolymers. U.S. Pat. No. 5,298,568 to Suzuki, issued Mar. 29, 1994, is directed to an adhesive useful in recycling of corrugated paper. The adhesive is a copolymer of an alpha-olefin and maleic anhydride. The copolymer may be modified by the addition of hydroxyl, amino, aziridinyl and mercapto groups.
Another adhesive, used to help bond polymers such as ethylene or vinyl alcohol copolymers to polyolefins and polyamides, is discussed in U.S. Pat. No. 5,115,033 to Wong, issued May 19, 1992, the adhesive contains a polymer and a catalytic agent. The polymer is a polyolefin grafted onto an unsaturated carboxylic acid or anhydride. The catalytic agent is a monoalkyl phosphate or alkylamine.
An anhydride polymer is part of an electrically conductive gel composition for use in establishing a low resistance contact between an electrode and a biological body in U.S. Pat. No. 5,178,143 to Kwak et al., issued Jan. 12, 1993. While this gel is not, strictly speaking, an adhesive, it is made from a crosslinked, neutralized copolymer of maleic anhydride and vinyl ether.
One problem with denture adhesive copolymers, particularly MVE/MA copolymers, is that covalent cross-linking is not an attractive approach to fine-tuning the desired properties of the denture adhesive. Covalent cross-linking can reduce the affinity of the copolymer for water, which can have an adverse effect on the properties of the copolymer for some important applications.
Cross-linking agents have been tried with denture adhesives, including propylene glycol and glycerin in U.S. Pat. No. 5,696,181 to Chang et al., issued Dec. 9, 1997.
Bioadhesives that may be useful as denture adhesive ingredients have also been made with anhydride copolymers. U.S. Pat. No. 5,066,709 to Chaudhuri et al., issued Nov. 19, 1991, is directed to a bioadhesive composition comprising a MVE/MA copolymer monofunctional lactam side groups.
One formulating a denture adhesive must consider organoleptic properties in addition to the functional aspects of a cross-linking agent. A certain amount of a cross-linking agent can be expected to hydrolyze in an aqueous environment such as the oral cavity, especially if the covalent cross-link is accomplished through an ester linkage. Thus, the cross-linking agent itself must be nontoxic at a minimum and should also not provide an unpleasant taste, texture or other sensation to the user.